The focus of this proposal is to identify the specificities of neutralizing antibodies (NAbs) against primary isolates of HIV-1 that ultimately may be associated with protection from HIV-1 infection in humans. The NAb response of two populations of seropositive individuals, the long-term survivors and recent seroconverters, are especially important to dissect in order to identify particular NAb responses that may be protective. Long-term survivors have recently been found to have a potent and broad NAb response against primary isolates, in distinct contrast to the lack of NAbs in subjects with progressive disease. Identification of the neutralization epitopes recognized by these individuals via mAb/Fab isolation/characterization should aid in vaccine design and may provide therapeutic mAbs/Fabs for treatment of less fortunate HIV-1 infected individuals. The NAb response of recent seroconverters is important to dissect because the control of viremia and its consequent symptoms during primary HIV-1 infection have been correlated with a longer time to develop AIDS. NAbs, though developed later than CD8+ cells capable of suppressing viral replication, may play a significant role in controlling the level of viremia in primary and later HIV-1 infection. The recent seroconverters offer an ideal opportunity to observe the temporal development of various NAb specificities to sequential viral isolates during early HIV-1 infection, particularly since the "seroconversion-type" virus from these individuals is relatively homogeneous in Env sequence and phenotype. In this application, we propose to isolate human monoclonal antibodies (mAbs) and/or Fab fragments with high affinity for autologous, oligomeric, primary isolate Env from recent seroconverters longitudinally and from long-term survivors of HIV-1 infection. Those mAbs/Fabs with potent neutralizing activity against autologous primary isolates of HIV-1 or, in the case of the long-term survivors, against heterologous primary isolates of HIV-1 will be selected for further characterizations including epitope mapping. Ongoing collaborative studies are focused on obtaining longitudinal clinical and virological data on these same individuals, and autologous, longitudinal serum samples will be assayed in parallel with anti-Env mAbs/Fabs which we isolate for neutralizing ability against autologous virus from different timepoints. These studies should allow us to begin identification of neutralization epitopes in primary isolates that are recognized by these two populations of individuals, to see whether these epitopes differ in the two patient populations, and to follow the temporal development of these specificities in recent seroconverters. Future studies may be designed to establish a definitive link between response to certain neutralization epitopes and protection in one or both of these populations and/or to extend these studies to other groups of individuals.